1. Field of the Invention
The present invention relates to a clamping mechanism for use within an injection molding machine for molding synthetic resins.
2. Description of the Related Art
A clamping mechanism for opening and closing a mold is disclosed in Japanese patent application Laid-Open No. 60-143915. The mechanism includes means for reducing hydraulic resistance during the opening and closing of the mold. In particular, an internal diameter of a front chamber of a cylinder is larger than an internal diameter of a rear chamber of the cylinder, and a clearance is formed between the peripheral wall of the rear chamber of the cylinder and a piston.
In the above-described clamping mechanism, hydraulic resistance within the cylinder is reduced because oil can flow through the clearance around the periphery of the piston. But a large quantity of oil is required when a clamping ram is moved at high speed. Conventional pipes for receiving and supplying oil have small inlets and outlets. Hence, flowing-in-and-out of oil during operation of a large clamping mechanism is not smooth, and high-speed opening and closing cannot be carried out.
In view of the foregoing, the present inventor has developed means, as disclosed in Japanese Patent Application No. 61-276185, in which a front chamber of a cylinder and a rear chamber of the cylinder communicate with each other through a large diameter passage which can be opened and closed by a valve, whereby oil from the front chamber can be moved to the rear chamber through the passage during opening and closing of the mold. Thus, the flowability of oil within a large clamping mechanism is not impaired by pipe resistance and high-speed opening and closing can be carried out smoothly. However, cavitation tends to occur in the back of the piston because of a shortage of oil within the cylinder. Such cavitation increases the amount of time needed to establish the final clamping pressure.
A supply tank has been used in the prior art to supply oil to a cylinder as a piston moves out of the cylinder. But such a tank requires a valve and means for coordinating the operation of the valve with the movement of the piston. Such an arrangement is complicated and difficult to operate.